FIG. 1A illustrates an exemplary miniature projector display device 100, sometimes referred to as a picoprojector. The miniature projector device 100 can be integrated with or attached to a portable device, such as, but not limited to, a mobile phone, a smart phone, a portable computer (e.g., a laptop or netbook), a personal data assistant (PDA), or a portable media player (e.g., DVD player). The miniature projector device 100 can alternatively be integrated with or attached to a non-portable device, such as a desktop computer or a media player (e.g., a DVD player), but not limited thereto. The miniature projector device 100 can also be used in television applications, digital picture frame applications, as well as other applications.
Referring to FIG. 1A, the projector display device 100 is shown as including a video source 102, a controller 104 (e.g., an application specific integrated circuit and/or a micro-controller), a laser diode driver (LDD) 108 and voltage regulators 110. Depending on the type of video source, a video analog-front-end (AFE) (not shown) can be included between the video source 102 and the controller 104, and the video AFE may include, e.g., one or more analog-to-digital converters (ADCs). However, a video AFE may not be needed where the video source is a digital video source. The controller 104 can perform scaling and/or pre-distortion of video signals before such signals are provided to the LDD 108. The voltage regulators 110 can convert a voltage provided by a voltage source (e.g., a battery or AC supply) into the various voltage levels (e.g., four voltage levels V1, V2, V3 and V4) for powering the various components of the projector display device 100. The voltage regulators 110 can include, e.g., four DC-DC converters, each one of which produces a different one of the four voltage levels V1, V2, V3 and V4.
The LDD 108 is shown as including three current output digital-to-analog converters DACs 1091, 1092 and 1093 (which can be collectively referred to as DACs 109, or individually can be referred to as a DAC 109). The LDD is also shown as including a serial interface 122 which may receive, via a serial bus 103, a serial enable (SEN) signal and a serial clock signal (SClk) from a serial interface of the controller 104. Additionally, a bi-directional serial data input/output (SDIO) line of the serial bus 103 allows the controller 104 to write data to and read data from registers within the LDD 108. Alternative serial buses and interfaces can be used, such as, but not limited to, an Inter-Integrated Circuit (I2C) bus or a Serial Peripheral Interface (SPI) bus and interface. The LDD 108 also includes registers, and the like, which are not shown.
The current output DACs 109 of the LDD 108 drive laser diodes 112, which can include, e.g., a red, a green and a blue laser diode, but are not limited thereto. More specifically, each DAC 109 drives a current through a laser diode to cause the laser diode to emit light. Such currents can be pulled through the laser diodes (as in FIG. 1A), or pushed through the laser diodes (as in FIG. 1B). Where the LDD 108 is used to drive a red (R), a green (G) and a blue (B) laser diode, the LDD can be referred to as a RGB triple laser diode driver. The use of alternative light emitting elements, such as light emitting diodes (LEDs), etc., is also possible. Accordingly, as the term is used herein, a laser diode driver (LDD), unless stated otherwise, can drive light emitting elements including, but not limited to, laser diodes (e.g., the LDD may alternatively drive LEDs).
The light produced by the laser diodes 112 or other light emitting elements can be provided to beam splitters 114, which can direct a small percentage of the light toward one or more calibration photo-detectors (PDs) 120, and direct the remainder of the light toward projector optics 116, which include lenses, mirrors, reflection plates and/or the like. The light output by the optics 116 can be provided to one or more micro mirror(s) 118. The mirror(s) 118 can be controlled by the controller 104, or another portion of the system, to raster-scan reflected light onto a surface, e.g., a screen, a wall, the back of a chair, etc. Alternatively, the light can be directed to a liquid crystal on silicon (LCoS) display that projects the light onto a surface, e.g., a screen, a wall, etc.
Video data typically contains three primary colors, red (R), green (G) and blue (B)—which in a digital system are sent as three color data words of some length, e.g., 10-bits each in the case of certain miniature projectors. All three color data words are transferred from the controller 104 to the LDD 108 for each pixel.
The voltage drop across each laser diode 112 (for a constant current) varies with temperature and aging. Traditionally, the laser diode supply voltage for a laser diode 112 (e.g., provided by a DC-DC converter type voltage regulator) is set for the worst case voltage drop across the laser diode 112 to ensure that there is enough voltage headroom available for the current output DAC 109 to drive the laser diode 112 under the worst case conditions. This leads to a higher supply voltage than is necessary for non-worst case conditions, and thus, wasted power. In a battery powered portable device, this is undesirable.
In FIG. 1A the DACs 109 are presumed to have NMOS or NPN output stages, and the outputs of the DACs 109 are shown as being connected to cathodes of the laser diodes 112 (with the anodes of the laser diodes 112 being connected to respective outputs of the voltage regulators 110). Alternatively, the DACs 109 can have PMOS or PNP output stages, and the outputs of the DACs 109 can be connected to anodes of the laser diodes 112 as shown in FIG. 1B (with the cathodes of the laser diodes 112 being connected to ground or another low voltage rail). In FIG. 1B, each DAC 109 receives a separate supply voltage produced by a separate voltage regulator 110. It is also possible that DACs include output stages that include a mixture of N-type and P-type devices.